Preheat oven to 375ºF. Butter a 2 1/2-quart casserole dish and set aside. Stir pasta from both boxes into a large saucepan of boiling water. Cook uncovered for 6 to 7 minutes (2 to 3 minutes less than noted on the box), until al dente. Drain the pasta and return to the saucepan. Stir in packets of cheese mix from both boxes, 8 tablespoons of butter and 1/2 cup of milk.